The present invention describes a cutting tool for metal machining, having a body of cemented carbide, cermet, ceramics or high speed steel and on the surface of said body, a hard and wear resistant refractory coating is deposited. The coating is adherently bonded to the body and covering all functional parts of the tool. The coating is composed of one or more layers of refractory compounds of which at least one layer consists of fine-crystalline alumina, Al.sub.2 O.sub.3, deposited by Physical Vapor Deposition (PVD) and the non-Al.sub.2 O.sub.3 layer(s), if any at all, consists of metal nitrides and/or carbides with the metal elements chosen from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al.
It is well known that for cemented carbide cutting tools used in metal machining, the wear resistance of the tool edge can be increased by applying thin, hard surface layers of metal oxides, carbides or nitrides with the metal either selected from the transition metals from the groups IV, V and VI of the Periodic Table or from silicon, boron and aluminium. The coating thickness usually varies between 1 and 15 .mu.m and the most widespread techniques for depositing such coatings are PVD and CVD (Chemical Vapor Deposition). It is also known that further improvements of the performance of a cutting tool can be achieved by applying a pure ceramic layer such as Al.sub.2 O.sub.3 on top of layers of metal carbides and nitrides (U.S. Pat. Nos. 5,674,564; 5,487,625).
Cemented carbide cutting tools coated with alumina layers have been commercially available for over two decades. The CVD technique usually employed involves the deposition of material from a reactive gas atmosphere on a substrate surface held at elevated temperatures. Al.sub.2 O.sub.3, crystallizes into several different phases such as .alpha.(alfa), .kappa.(kappa) and .chi.(chi) called the ".alpha.-series" with hcp (hexagonal close packing) stacking of the oxygen atoms, and into .gamma.(gamma), .theta.(theta), .eta.(eta) and .delta.(delta) called the ".gamma.-series" with fcc (face centered cubic) stacking of the oxygen atoms. The most often occurring Al.sub.2 O.sub.3 -phases in CVD coatings deposited on cemented carbides at conventional CVD temperatures, 1000.degree.-1050.degree. C., are the stable alpha and the metastable kappa phases, however, occasionally the metastable theta phase has also been observed.
The CVD Al.sub.2 O.sub.3 coatings of the .alpha.-, .kappa.- and/or .theta.-phase are fully crystalline with a grain size in the range 0.5-5 .mu.m and having well-facetted grain structures.
Deposition at a typical temperature of about 1000.degree. C. causes the total stress in CVD Al.sub.2 O.sub.3 coatings on cemented carbide substrates to be tensile, in nature. The total stress is dominated by thermal stresses caused by the difference in thermal expansion coefficients between the substrate and the coating, less intrinsic stresses which have there origin from the deposition process itself and are compressive in nature. The tensile stresses may exceed the rupture limit of Al.sub.2 O.sub.3 and cause the coating to crack extensively and thus degrade the performance of the cutting edge in particularly in certain applications, such as wet machining where the corrosive chemicals in the coolant fluid may exploit the cracks in the coating as diffusion paths.
Generally CVD-coated tools perform very well when machining various steels and cast irons under dry or wet cutting conditions. However, there exists a number of cutting operations or machining conditions when PVD-coated tools are more suitable e.g. in drilling, parting and threading and other operations where sharp cutting edges are required. Such cutting operations are often referred to as the "PVD coated tool application area".
Plasma assisted CVD technique, PACVD, makes it possible to deposit coatings at lower substrate temperatures as compared to thermal CVD temperatures and thus avoid the dominance of the thermal stresses. Thin Al.sub.2 O.sub.3 PACVD films, free of cracks, have been deposited on cemented carbides at substrate tempertures 450.degree.-700.degree. C. (DE 41 10 005; DE 41 10 006; DE 42 09 975). The PACVD process for depositing Al.sub.2 O.sub.3 includes the reaction between an Al-halogenide, e.g. AlCl.sub.3, and an oxygen donor, e.g. CO.sub.2, and because of the incompletness of this chemical reaction, chlorine is to a large extent trapped in the Al.sub.2 O.sub.3 coating and its content could be as large as 3.5%. Furthermore, these PACVD Al.sub.2 O.sub.3 coatings are generally composed of, besides the crystalline alfa- and/or gamma-Al.sub.2 O.sub.3 phase, a substantial amount of amorphous alumina, which in combination with the high content of halogen impurities, degrades both the chemical and mechanical properties of said coating, hence making the coating material less desirable as a tool material.
There exist several PVD techniques capable of producing refractory thin films on cutting tools and the most established methods are ion plating, DC- and RF magnetron sputtering, arc discharge evaporation, IBAD (Ion Beam Assisted Deposition) and Activated Reactive Evaporation (ARE). Each method has its own merits and the intrinsic properties of the produced coatings such as microstructure/grain size, hardness, state of stress, intrinsic cohesion and adhesion to the underlying substrate may vary depending on the particular PVD method chosen. Early attempts to PVD deposit Al.sub.2 O.sub.3 at typical PVD temperatures, 400.degree.-500.degree. C., resulted in amorphous alumina layers which did not offer any notable improvement in wear resistance when applied on cutting tools. PVD deposition by HF diode or magnetron sputtering resulted in crystalline .alpha.-Al.sub.2 O.sub.3 only when the substrate temperature was kept as high as 1000.degree. C. (Thornton and Chin, Ceramic Bulletin, 56 (1977) 504). Likewise, applying the ARE method for depositing Al.sub.2 O.sub.3, only resulted in fully dense and hard Al.sub.2 O.sub.3 coatings at substrate temperatures around 1000.degree. C. (Bunshah and Schramm, Thin Solid Films, 40 (1977) 211).
With the invention of the bipolar pulsed DMS technique (Dual Magnetron Sputtering) which is disclosed in DD 252 205 and DE 195 18 779, a wide range of opportunities opened up for the deposition of insulating layers such as Al.sub.2 O.sub.3 and, furthermore, the method has made it possible to deposit crystalline Al.sub.2 O.sub.3 layers at substrate temperatures in the range 500.degree. to 800.degree. C. In the bipolar dual magnetron system, the two magnetrons alternately act as an anode and a cathode and, hence, preserve a metallic anode over long process times. At high enough frequencies, possible electron charging on the insulating layers will be suppressed and the otherwise troublesome phenomenon of "arcing" will be limited. Hence, according to DE 195 18 779, the DMS sputtering technique is capable of depositing and producing high-quality, highly-adherent, crystalline .alpha.-Al.sub.2 O.sub.3 thin films at substrate temperatures less than 800.degree. C. The ".alpha.-Al.sub.2 O.sub.3 layers", with a typical size of the .alpha.-grains varying between 0.2-2 .mu.m, may also partially contain the gamma(.gamma.) phase from the ".gamma.-series" of the Al.sub.2 O.sub.3 polymorphs. The size of the .gamma.-grains in the coating is much smaller than the size of the .alpha.-grains. The .gamma.-Al.sub.2 O.sub.3 grainsize typically varies between 0.05 to 0.1 .mu.m. In the Al.sub.2 O.sub.3 layers where both modifications of .gamma.- and .alpha.-phase were found, the .gamma.-Al.sub.2 O.sub.3 phase showed a preferred growth orientation with a (440)-texture. When compared to prior art plasma assisted deposition techniques such as PACVD as described in DE 49 09 975, the novel, pulsed DMS sputtering deposition method has the decisive, important advantage that no impurities such as halogen atoms, e.g. chlorine, are incorporated in the Al.sub.2 O.sub.3 coating.